Because the interaction between myosin and ATP has a central role in muscle contraction, it is necessary to elucidate this interaction before an understanding of the molecular basis of contraction can be achieved. One objective is to correlate the conformational changes in myosin which accompany the hydrolysis of ATP with the intermediate steps in hydrolysis, and to determine which steps may be important in regulating the rates of ATP hydrolysis and contraction. To accomplish this we will determine the effects of ATP and other nucleoside triphosphates on the ultraviolet absorption of heavy meromyosin or HMM that has been selectively spin labeled at the S1 thiol groups. The kinetic parameters associated with myosin and actomyosin catalyzed hydrolysis of ATP will be studied; these parameters will include the initial rapid burst of phosphate liberation, the dissociation of products of hydrolysis and the spectral changes which occur during hydrolysis of ATP. Stopped flow methods will be used to study very rapid changes. These studies will be extended to myosins from "red" skeletal muscles and cardiac muscle which have slower velocities of contraction than the "white" skeletal muscles.